In a fuel cell system, a fuel gas such as hydrogen and an oxidizer gas containing oxygen are electrochemically reacted across an electrolyte to produce electrical energy. Conventional fuel cell systems contain one or more unit fuel cells including generally a fuel electrode coated with a redox catalyst layer, an oxidizing electrode coated with a redox catalyst layer, and an electrolyte film separating the electrodes and having a gas passage formed to supply oxidizer gas (e.g. air) to the oxidizing electrode (e.g. cathode) in an oxidizer electrode chamber, and fuel gas (e.g. hydrogen) to the fuel electrode (e.g. anode) in a fuel electrode chamber.
Conventional fuel cell systems may include pipes for supplying and exhausting oxidizing gas and fuel gas to and from the fuel cell stack. However, since only a portion of the oxidizing gas and the fuel gas supplied to the fuel cell generally reacts within the respective oxidizing electrode chamber and fuel electrode chamber, the gas exhausted from a fuel cell stack may be rich in unreacted fuel gas and/or oxidizer gas. This leads to fuel cell operating inefficiency and waste of expensive fuel gas.
Although various methods have been developed to monitor the exit concentration of the exhaust gas from an operating fuel cell and adjust the operating parameters of the fuel cell system to achieve higher fuel gas conversion, these methods generally require expensive gas concentration detection instrumentation that adds substantially to the cost of a fuel cell system. Moreover, the existing gas concentration monitoring systems may suffer from operating instability and gas detection interferences. For example, gas detection monitors which operate by monitoring the thermal conductivity or infrared absorbance of the exhaust gas may respond non-selectively to gases other than the fuel gas, for example, water vapor or carbon dioxide. This may provide a false indication of the fuel gas concentration in the fuel cell system exhaust and lead to selection of improper fuel cell operating parameters. Selection of improper fuel cell operating parameters can seriously degrade fuel cell operating efficiency, long term cycling performance and fuel cell life.